Display panels incorporating 7-segment (7-bar figure-of-eight) and 8-segment (7-bar figure-of-eight+decimal point) display units are well known, and used both in calculators and digital instruments. Low-power consumption liquid crystal medium display panels are also well known, particularly those using the twisted nematic effect or the dynamic scattering effect.
Two forms of panel address are also well known, discrete address and time multiplexed address. For direct discrete address, the 7-segment display requires seven discrete front plane electrodes for each display unit and a common back-plane electrode. Where more than a few display units are required, the number of connections to the panel electrodes becomes prohibitively large. In general, the larger the number of connections, the lower is the production yield, and the higher is the production cost. This form of address therefore has very restricted application. On the other hand the number of connections required for time multiplexed address can be considerably smaller. For this the display segments of each display unit are defined by seven shaped electrodes front and back, various of these shaped electrodes being interconnected (eg GB Patent Specification No. 1596 705). However, in this technique selected segments are driven ON for only part of the address signal cycle, and the driving fields or voltages are changed between a maximum level and a minimum but finite level to drive the segments either ON or OFF. The ratio of maximum to minimum level is however limited and depends on the time multiplexed technique adopted. Furthermore the minimum level must be of sufficiently low level that segments are not inadvertently driven ON. Many liquid crystal media exhibit a low threshold above which the media changes to an ON state, a threshold often sensitive to temperature. Thus to optimize maximum level it is often necessary to incorporate electronic compensation for temperature change, accepting increase in unit costs. Certainly it is difficult, if not impossible, to achieve the angle of view, brightness, and contrast performance achieved by direct discrete address.
Panels incorporating dyed phase change liquid crystal media can provide attractive displays with excellent angle of view, good contrast, and reasonable brightness. Also for different color choice, a wide range of dyes can be used; there is a wide choice. However, these media usually have a rapid response, and exhibit substantial hysteresis--ie the voltages required to turn OFF segments ON, and ON segments OFF, can differ markedly, and the acceptable drive level minimum can be exceptionally low, making time multiplexed address techniques impractical.
Brief mention is made here of display panels, panels other than multi-segment display panels, that utilize isogonal signals for their address. GB Pat. No. 1,526,266 discloses a matrix display panel using three isogonal address signals, in particular three sinusoidal signals of like frequency and amplitude but which differ from each other in phase by .+-.120.degree.. This panel may be used to display characters--for example the letter "L", but to do this the signals must be applied to the electrodes in a defined time sequence. The displayed character "L" is depicted by selected matrix elements that are held "OFF" against a background of all other matrix elements that are held "ON". It is a disadvantage that representative elements are held "OFF" for only part of the time, thus though to the observer's eye all representative elements may appear to be OFF simultaneously, the contrast is less than optimum. Better contrast is obtainable using the address technique disclosed in GB Patent Application No. 2,001,794A, a technique using pseudo-random binary sequency (p.r.b.s.) coded address signals. According to this technique a different p.r.b.s. signal is applied to each of the matrix row electrodes, and selected row signals are applied to the matrix column electrodes. Thus all matrix elements, except one selected in each column, are driven "ON". This technique is useful for displaying single valued waveforms, but since only one element per column may be used for display representation, this technique is not readily applicable to character display. P.r.b.s. coded address signals have also been used for index pointer displays--eg for clock and meter displays--cf GB Patent Application No. 2,044,975A.